The field of wireless technology is currently undergoing a revolution and experiencing phenomenal growth. Mobile phones, once considered a novelty and referred to as xe2x80x9ccar phones,xe2x80x9d are now ubiquitous. A batch of wireless personal digital assistants, phones, and computer peripherals are entering the market, with wireless Internet access as a driving force. Wireless communications is one of the most important technology sectors of the our economy. This invention provides circuits and methods for a key building block in this marketplace.
Wireless devices typically transmit and receive data through the air using high-frequency electromagnetic waveforms. Data transmission is begun by encoding the data to be transmitted. The encoded data is used to modulate (i.e., is multiplied by) a high frequency carrier signal. This results in a signal having frequency components at the sum and difference of the frequencies of the multiplied signals. The low frequency difference component is filtered, and the high frequency sum component, the modulated carrier signal, is applied to an antenna for transmission. The transmitted signal is referred to as a radio frequency (RF) signal.
Reception involves receiving an RF signal on an antenna and filtering undesired spectral components. The signal is demodulated by multiplying it with a local oscillator (LO) signal having a frequency approximately equal to the frequency of the carrier signal. Again, the result is a signal having frequency components at the sum and difference of the frequencies of the LO and RF signals. The high frequency sum component is removed, while the low frequency difference component is the data signal, which is decoded.
The proper generation of the carrier and LO signals is of critical importance. Spurious frequency components, noise, and jitter distort the data signal and degrade reception. Also, the LO signal at the receiver needs to be tuned near the carrier""s frequency. Too large a tuning range leads to phase noise, and too small a range may mean a receiver cannot be properly tuned. Thus, it is desirable to have circuits and methods for generating carrier and LO signals with high spectral purity, low phase noise and jitter, and proper tuning range.
An exemplary embodiment provides a voltage controlled oscillator (VCO) that may provide either or both carrier and local oscillator signals. A tuning voltage is applied to the VCO through a resistor to a varactor diode. The resulting capacitance of the varactor diode is AC coupled through another capacitor to a VCO tank circuit. The control voltage may be analog or continuous, as from a phase-locked loop (PLL), or digital, as from a trimming circuit In an exemplary embodiment, a plurality of tuning diodes, and both continuous and digital tuning are used.
AC coupling capacitors isolate the tank circuit from the varactor diode capacitances. This means that both the VCO output and control voltages can vary through the entire supply range without forward biasing the varactor diodes. Also, series resistances found in prior art variable capacitance networks are reduced, thus maintaining a high Q for the VCO tank circuit, which results in good start-up performance and low phase noise. Further, the capacitive coupling reduces the oscillator""s Kvco resulting in reduced sensitivity to noise on the VCO control voltages.
Another exemplary embodiment provides a method of tuning a voltage controlled oscillator. The method includes measuring a frequency of oscillation of the voltage controlled oscillator, comparing the frequency of oscillation to a desired frequency, generating a logic signal, and applying the logic signal to a resistor. The resistor is coupled to a fit capacitor and a second capacitor, the first capacitor is coupled to an inductor, and the second capacitor is coupled to a first supply terminal.
A further exemplary embodiment provides an integrated circuit. The integrated circuit has a VCO which includes a first inductor, a first capacitor coupled to the first inductor, a first varactor diode coupled to the first capacitor, and a first isolation resistor coupled to the first capacitor and the first varactor diode. The first isolation resistor is configured to receive a control voltage.
Another exemplary embodiment of the present invention provides an integrated circuit having a voltage controlled oscillator. The VCO includes a first inductor, a second inductor, a first capacitor coupled to the first inductor, a second capacitor coupled to the first inductor, a third capacitor coupled to the first capacitor, and a fourth capacitor coupled to the second capacitor. Also included are a first isolation resistor coupled to the first capacitor and the third capacitor, wherein the first isolation resistor is configured to receive a control voltage, a second isolation resistor coupled to the second capacitor and the second varactor diode, wherein the second isolation resistor is configured to receive the control voltage, a first device having a drain coupled to the first inductor and a gate coupled to the second inductor, and a second device having a drain coupled to the second inductor and a gate coupled to the first inductor.
Yet a further exemplary embodiment provides a phase-locked loop. The phase-locked loop includes a phase detector configured to receive a reference clock, a low-pass filter coupled to the phase detector, a voltage-controlled oscillator coupled to the low-pass filter, and a divider coupled between the voltage-controlled oscillator and the low-pass filter. The voltage-controlled oscillator includes a first inductor, a second inductor, a first capacitor coupled to the first inductor, a second capacitor coupled to the first inductor, a third capacitor coupled to the first capacitor, and a fourth capacitor coupled to the second capacitor. Also included is a first isolation resistor coupled to the first capacitor and the third capacitor, wherein the first isolation resistor is configured to receive a control voltage, a second isolation resistor coupled to the second capacitor and the fourth capacitor, wherein the second isolation resistor is configured to receive the control voltage, a first device having a drain coupled to the first inductor and a gate coupled to the second inductor, and a second device having a drain coupled to the second inductor and a gate coupled to the first inductor.
A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings.